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Ok, a little backstory- the truck has a build date of 8/99, auto 2500 with 370k miles 

I bought this truck recently with a bad transmission but had a reman in the bed. I dropped the reman off at a tranny shop to have them check it out and picked up a single disc torque converter while I had it out. The tranny checked out ok so I swapped the transmissions. On initial install, I only had 1,2,3 shifts with no od and no lockup. I dropped the pans on both and noticed the pressure sensors were different. I swapped the od solenoid, pressure sensor, and wiring harness from the original tranny to my new transmission. By doing so, I was able to pick up od and lockup but not it will not downshift from 3rd gear unless I do it manually. I bring it to the tranny shop to have them read the pressures and figure out what’s going on before I just start throwing parts at it. They were never able to get the scanner to read anything from the obd port. 
This brings me to my current issue. I have power and grounds on the port. I checked resistance and the violet and blk/white wires from the cluster to the pcm and they all check out good. I’m stumped as to where to go from here. The reason I bring up both issues is I’m wondering if they could be related? I’m leaning towards the pcm being bad but I don’t want to just throw one at it and see what happens. Does anyone have anymore diagnosis tests for the obd port? Or any insight whatsoever? Thank you 



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There is two bus connections if you look at the wire map. There is the CCD bus then there is SCI bus that only access the PCM and ECM. Double check those connections. Beyond that there is only power and ground. 


+12V at pin 16

GND at pin 5

GND at pin 4


CCD at pins 3 and 11

SCI at pins 6, 7 and 14



ccd network wiring

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Thanks for all your help with Mike. I was able to check the resistance on all the wires today and everything looks good there. I checked all the ccd through the gauge cluster, pcm, and ecm. Same with the sci. Everything there looked good with the only assumption that I shouldn’t have anything at the sci bus on d20 since I have a diesel. (The wiring is there. ) All of my power and grounds are correct and the voltages are correct at the ccd pins on the obd port. I was able to borrow a 2001 pcm from a buddy today and swapped out real quick to no avail. My code reader lights up but no communication. 
Any ideas?


 Thank you, Jim 


Edited by Jsully72
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